This invention relates to a sensor for sensing the ground speed of a vehicle, such as an agricultural vehicle.
Doppler-type ground speed sensing systems are well known. The velocity, v, can be determined from frequency shift, .DELTA.f, by a modified Doppler equation: ##EQU1## where c is the signal or wave propagation speed, where .alpha. is the angle of the transmission axis from horizontal, and where f(t) is the transmitted signal frequency. Conventional Doppler radar systems include a homodyne detector which mixes the transmitted signal with the received signal, thereby producing a frequency which is proportional to the absolute value of the difference between the transmit and receive frequencies. With such systems, it is impossible to distinguish between forward and backward motion. Furthermore, at low speeds, fluctuations due to waving of grass or vehicle pitching can cause the average indicated speed to be greater than the average actual speed. Another problem with homodyne-type systems is that the response time of the output is proportional to ground speed (frequency shift). At low vehicle speeds, this slow response time may cause inaccuracies. Finally, such a system may yield ambiguous information because a zero frequency output signal from the receiver may be the result of zero ground speed, or it could be the result of a signal "dropout" condition.